Telephone system



June 10, 1941.. l.. G. scHlMBF TELEPHONE SYSTEM Filed Lmy fr; 1939 /M/EA/ TOR L. G. 5CH/MPF A TTORA/FV Patented June 1Q, 1941 UNETED 23eme@ S'i`A'`ESv PATENT @FFME 'receptions SYSTEM application Juny i, i939, serial Nc. 283,149

' (ci. we is) ii Claims.

This invention relates to telephone systems and particularly to those systems in which automatic switches are used for establishing connections.

The objects of the invention are to eliminate some of the individual equipment usually provided for subscribers lines; to enable the automatic switches to select a subscribers line by means of a simple and positive test; to provide a more reliable test for controlling the stepping action of the switch while hunting for a calling subscribers line; and to otherwise improve systems of this character.

It has been proposed heretofore to eliminate the individual line and cut-orf relays and to replace them by a relay mechanism common to a group of lines. To accomplish this, however, it would be necessary to provide some sort of switching mechanism for associating the common relays with any one of the lines when calling.

It has also been proposed to use a vacuum tube in common to a group of lines and to connect the control element through individual resistances to the respective subscribers lines, thus doing away with the individual line relays. An arrangement of this kind, however, would be subject to false I operation due to the combined eiect of several line leaks, and would not lend itself readily to a positive and reliable test that would enable the line finder to distinguish and seize the calling line in the group.

` According to the present invention, these difliculties are overcome by means of an electrical circuit in which a space discharge device having a multiplicity of control electrodes is provided in common to a group of subscribers lines and in which a pair of space discharge devices are provided for each line. The control electrodes of the group device are individual, respectively, to the subscribers lines and the electrical condition of each is altered in response to the discharge of the pair of line devices when a call is initiated. This change in the electrical condition of one of the control electrodes causes the discharge of the group device which, in turn, causes the starting of a line finder to hunt for the calling subscribers line.

Another feature of the invention is a testing arrangement in which, as a result of the closure of the line loop at the substation and the discharge of the pair of line devices in consequence thereof, a potential is applied to the test terminal of the line in the iinder switch bank at the same time that the potential of the individual control electrode is altered. This calling potential on the terminal of the iinder bank permits fil) the switch to distinguish the calling line from the non-calling lines.

Another feature of the invention is a line-finder switch in which a space discharge device is used in circuit with one of the space discharge devices of the calling line whereby the potentialapplied to the test terminal by the latter is used for operating the former when the brushes of the switch reach the desired terminal in the desired level. This arrangement makes for a quick and accurate test.

The foregoing and other features of the invention will be described in detail in the following specification which should be taken in conjunction with the accompanying drawing.

The drawing represents, schematically, a telephone system in which the features of the in- Vention are embodied and which employs automatic switches of the well-known two-motion step-by-step type having two sets of terminal banks. It is to be understood, however, that the embodiment of the invention in this particular system is given by Way of illustration only, and that the invention may be applied to automatic telephone systems employing other types of switches.

Referring to ther drawing, there are shown subscribers stations A and B, the associated line circuits LC and LC', a line-finder switch LF, a selector switch S, a connector switch C, and a called subscribers station D;

The` subscribers stations A and B which appear on corresponding levels in the upper and lower terminal banks to which the line-ndcr switches have access are provided with the usual subscribers set, which is equipped with a dial for controlling the establishment of the desired connections. The selector switch S and the connector switch C are of the well-known step-by-step or Strowger type, and reference may be had to pages 53l to 67 inclusive of the 2nd edition of Automatic telephony by Smith and Campbell, for a detailed description of the operation of the circuits associated with these switches. Only those portions of the circuits of the selector and connector switches are shown as are required for a clear and complete understanding of this invention, the omitted portions of these circuits being indicated by broken lines.

, The line circuits LC and LC and the lineender switch LF are shown in detail since the features of the invention apply speciiically to these circuits. Each line circuit, as for instance line circuit LC, is provided with two cold cathode gaseous discharge tubes |02 and |35, one foreach side m3' and |04 of the line, while each of two groups of lines, one appearing on the terminals of a level in the upper bank of which line Ais representative, and the other appearing in the terminals of a corresponding level in the lower bank of which line B is representative, is provided with a cold gaseous discharge tube |03 having a number of control electrodes such as electrodes |53 and |57.

Cold cathode gaseous discharge tube |08 having a multiplicity of control electrodes, such as electrode |56, is associated with the two groups of line circuits, that is, the group of lines appearing in one terminal level in the upper bank, of which line A is representative, and the group of lines appearing in a corresponding terminal level in the lower bank of which line B is representative. Tube |08 is of the type which breaks down and becomes conducting when the potential on any one of its control electrodes becomes sufficiently positive with respect to the cathode potential of the tube. The line tubes |02 and |05 are of the type which break down and become conducting upon the application of a definite breakdown potential between its two electrodes.

Each individual line circuit of a group, such as line circuit LC, for example, connects to one of the control electrodes of tube |08 Via the line tube |02 and when a call is originated by a subscriber, a potential change on the control electrode associated with that line circuit and caused by the breakdown of tube |02 causes tube |08 to break down and operate means whereby the line-nder switch LF is caused to hunt for the calling line. The breakdown of the line tube |02 also places an identifying potential on the test terminal of the calling line to enable the line iinder to select the terminals associated with this line.

Each line circuit connects to a set of four terminals appearing in the terminal banks of the line-finder switch, as indicated by the individual line circuit LC which connects to terminals ||8, H0, |20 and |2|. The line-finder switch LF is of the well-known step-by-step type provided with two banks of one hundred sets of terminals each, such as the two sets composed of terminals H8, ||9, |20 and |2| on the upper level Ato which line A is connected and terminals |58, |50, |50 and |6| on the lower level to which line B is connected. 'Ihese terminal sets are arranged in ten Vertical levels in each bank, each level consisting of ten horizontal terminal sets, and are selectable in the upper bank by brushes |22, |23, |24 and |25, and in the lower bank by brushes |33, |34, |35 and |35 which, by means of a common shaft (not shown), are advanced vertically under the control of vertical magnet |3| to the proper level and then horizontally under the control of rotary magnet |30 to the terminals of the calling line. The line nder is provided with a commutator |26 that has ten conducting segments, one segment for each two corresponding levels of the linender bank terminals. Associated with this cornmutator is a brush |31 which is mounted on the common brush shaft (not shown) and which advances simultaneously with brushes |22, |23, |24 and |25 and |33, |34, |35 and ISS-during vertical stepping. Relays |44 and |43 are connected, respectively, to cold cathode gaseous discharge tubes |40 and |39 and one of these tubes breaks down and becomes conducting when the calling line terminals, on their respective levels, are encountered, each causing thereby the operation of circuits that immediately arrest furd ther rotary magnet operations.

The operations of these circuits will now be fully set forth in detail in the following description of a call from station A to station D.

Negative battery |00 is normally applied through high resistance |0| to the cathode of tube |02 and positive battery |01 is normally applied through the two branches of high resistance |06 to the anode of tube |05. When the subscriber A removes his receiver from the switchhook to initiate a call, the line loop is closed in consequence of which the anode of tube |02 and the cathode of the other tube |05 are connected together to form one metallic conductor intermediate the dielectric medium which pervades the two separate electrodes connected, respectively, to the sources of potential |0| and il. The additive effective of the two sources of potential is suiicient to break down the dielectric, resulting, in turn, in the ionization of the gases within the two tubes as a consequence of which they become conducting and cause a current to flow through the line loop that produces a voltage drop in high resistance Il which is applied to the test terminal ||8 of the line as a test potential that the line is calling, and another voltage drop in resistances Il, ||5 and ||4 which is applied to the control element |56 of multielement tube |08.

The cathode 09 of tube |08 is connected to a source of negative potential and the anode I l0 is connected to a source of negativepotential through the secondary of transformer serially through the bridge formed by the winding of relay ||2 and condenser The primary winding of the transformer is connected to a source IIB of low frequency current. The potential diierencebetween the anode and the cathode of tube |08 while insuicient to cause the tube to break down, is sufcient to maintain ionization therein once it is broken down. Each control element of the tube |08 is connected to its own condenser ||3 intermediate the two series resistances ||4 and ||5 while the othei` side of the condenser is connected to negative battery. Normally, and when the associatedrline is not calling, there is negative battery connected to each side of the condenser, the other side having negative voltage applied thereto through resistance |0|. When, however, the line loop is closed, the value of the potential applied to the side connected to resistances ||4 and H5 becomes equal to the drop through resistances |0| and 5, whereupon the condenser begins to charge and the application of positive potential to the connected control element |56 is delayed until the condenser is fully charged to the potential difference between negative battery and the drop through resistances |0| and The purpose of this delay. is to prevent any operation of the tube |08 due to momentary surges and other transients, and the capacity of the condenser ||3 may be adjusted to the value required to provide a time of charge equal to the longest surge duration. If the potential Von conductor |45 persists, say, for a longerperiod than that required to charge the condenser 4| I3, then the control -element Will be chargedto -the breakdown potential of the tube through resistance H4.

Mention has been made of the fact that the primary winding of transformer is connected to the source ||6 of a low. frequency current. VWhen a negative half cycle of the volt- 'age wave is induced in the secondary winding, it will be added to the negative voltage of the `battery connected thereto. Since the tube will not nre so long as negative potential is connected to the anode thereof, the presence of positive potential on the control element |555 of line A when said line calls will have no eiect in breaking down the tube IS until a positive voltage is induced in the secondary winding of transformer |II, at which time the total effect of the combined induced voltage and that of the battery voltage will be the application of positive voltage to the anode of the tube |98. With one of the control elements at positive potential, the tube breaks down and becomes conducting, establishing a current now from battery, cathode |09, anode I I9, winding of relay I|2, secondary Winding of transformer III, battery. Relay ||2 operates in this circuit. As the induced voltage changes from positive to negative, due to the operation of source IIS, tube |98 will cease to be conducting when the induced voltage reaches its negative peak, at which time current will cease to new therethrough and, also, through relay H2. This relay, however, is made sufficiently slow to release so as to hold up during the negative half cycle of the potential wave. This may be done in any suitable manner as, for instance, by the use of a copper slug on the core of the relay or, as illustrated, by the connection across its winding of a condenser |I1 which charges on the establishment of current now through the tube and discharges through the winding of the relay when the current flow ceases, thereby tending to hold the relay up in the interval between successive positive potential waves that cause the tube |98 to nre repeatedly so long as the positive potential drop through resistance |14 is applied to the control element |56, which condition will prevail until the line iinder LF establishes connection with the terminals of the calling line, as will be described shortly.

Assume that terminals |I8, H9, |23 and I2I associated with the line circuit LCv of the calling station A are the fifth set of terminals in the fourth level of `the upper terminal banks of the finder switch. Tube |08 is common to all of the ten lines appearing in the fourth level of both terminal banks, each of these lines connecting respectively to one of the twenty control electrodes of the tube. Relay II2 is, therefore, common to all twenty lines in the fourth terminal levels and functions as a group start relay for all lines in these two levels. Relay II2 operating upon the aforementioned breakdown of tube |08 in response to the initiation of the call at station A, connects ground over its outer contacts to the fourth conducting segment of commutator |26 which corresponds to the fourth level of the line switch terminal banks, and connects ground through its inner contacts and through the outer contacts of relay |21 to battery through the winding of relay |28.

Relay |28, in operating, completes a circuit for relay |29 extending from ground through its No. 2 contacts, interruptor contacts of rotary and vertical magnets |30 and I3I, respectively, winding of relay |29 to battery. Relay |29, in operating, operates the Vertical stepping magnet |3I over the following path: ground through the contacts of relay |29, No. 2 back contacts of relay |32, winding of vertical magnet ISI, inner contacts of relay |21 to battery. rThe vertical magnet takes a step and, in so doing, causesf the line-nder brush sets |22, |23, I2|| and |25 and |33, |34, |35 and |36 to make one vertical step. In opening its interrupter contacts, vervtical magnet |3| further opens the circuit `of relay |29 which releases, in turn opening the circuit of the vertical magnet, causing it to fall back and reclose its interruptor contacts. The circuit of relay |29 is now reclosed and, upon the reoperation of this relay, the previously described circuit of vertical magnet |3| is once more made effemive whereupon the magnet operates to advance the line-nder brush sets another step and again to open its interruptor contacts, whereupon the circuit of relay |29 is again opened, consequent to which the relay releases to cause the release of the vertical magnet and the reclosure of its interruptor contacts. These interacting operations between vertical magnet IBI and relay |29 will continue, causing the brush shaft to be raised a corresponding number of steps, until relay |32, the circuit of which is controlled over brush |21, operates. On the first Vertical step of the brush sets, brush |31 contacts the fnst conducting segment of commutator |26 but, since this segment is open, no circuit path is completed for relay |32, and the same thing will be true for the next three steps sincey the segments corresponding to the second. and third levels will likewise be devoid of ground connections. However, when the brush |31 contacts thefourth segment, the ground connected thereto over the outer contacts of relay II2 causes a circuit to be completed through brush |31, segment |38, lower winding of relay |32, winding of relay |29 to battery. In this circuit, relay |32 operates and relay |29 is held, the former locking in a circuit extending from battery through the inner contacts of relay |21, winding of vertical magnet |3|, upper winding of relay |32 and the No. 3 contacts thereof, to ground on the No. 2 of relay |28. Relay |29, in its operated condition, extends ground over its contacts through the No. 2 front contacts of relay |32 to the winding of rotary magnet |39. The quantity of current owing through the holding circuit of relay |32 and, therefore, through the winding of vertical magnet Il, is not suiitcient to effect the operation of magnet ISI to open its interruptor contacts. Hence when the rotary magnet I3!) operates for the rst time, its circuit being completed to battery on the inner contacts of relay |21, the brush shaft is rotated in consequence of this operation and causes the engagement of the brush sets with the rst terminals in the respective levels to which they have been stepped and the disconnection of brush |31 from the segment |38. Relay |52, however, will not release because of its locking circuit and relay |29 will hold through the interrupter contacts of magnets |3| and |39 to ground on the inner contacts of relay |28. Upon the completion of the first rotary step, rotary magnet |39 opens its interrupter contacts and relay |29 releases whereupon ground is disconnected from the winding of the rotary magnet, causing it to release and reclose its interrupter contacts. The circuit of relay |29 is now closed again and the reoperation of this relay again closes the rotary magnet circuit causing it to operate and rotate the line-finder brushes into engagement with the second set of terminals on their respective levels. This interaction between the operation of rotary magnet |39 and relay |29 will continue until the brushes are rotated into engagement with the fth set ci terminals, at which time relay |42 is caused to operate over a circuit presently to be described, thereby causing the operation of relay |21 over an obvious circuit and the consequent removal of battery from the circuit of the rotary magnet. Relay |28 is caused to release in response to the operation of relay |21 and ground is removed from the circuit of relay |29. V

It will be observed that the line-finder circuit is provided with two gaslled three-element tubes, namely, tubes |39 and |40, the anode of the former being connected to a source of positive potential through the No. 2 contacts of relay |4| and resistance |48, and the anode of the latter to a source of positive potential through the No. 7 contacts of relay |42 and resistance |41. 'I'he cathode of tube |39 is connected to a source of negative potential through the winding of relay |43 and its control electrode is connected, via resistance |49, to brush |33 of the lower set of brushes. On the other hand, the cathode of tube |40 is connected to a source of negative potential through the winding of relay |44 and its control electrode is connected, via resistance |48, to brush |22 of the upper set of brushes.

When the brush sets |22|`25 and |33|36 engage the iirst terminal sets on `their respective fourth le 'els, upper brush |22 engages sleeve terminal H8 and lower brush |33 engages sleeve terminal |58. It has been assumed, however, that the calling line A is the fifth line on the upper fourth level so that brush |22 and brush |33, in testing the rst four sleeve terminals on their respective levels, will iind these terminals having negative battery through resistances and lill1 respectively applied thereto if the corresponding lines are idle. If a line is busy on an outgoing call, negative battery through low resistance |52 will be connected to test terminal 8 by the connecting line nder, via the No. 2 contacts of relay |42 which will be operated therein, No. 1 back contacts of relay i4! to brush |22 and terminal |'|8. This negative battery, extending through the multiple and applied to the control electrode of tube |40 will not affect said tube any more than battery through resistance |0| does. But whenv the brush |22 engages terminal ||8 of thecalling line A, the potential drop through resistance 0| will be connected thereto and will be applied to the control electrode of tube |40 vla resistance |48. The value of the potential drop, being positive with respect to the negative potential on the cathode of the tube, causes said tube to break down and Vestablish a circuit therethrough between negative battery through the winding of relay |44, space current between the cathode and anode of tube |40. No. '1 contacts of relay |42, resistance |41 to positive battery. Relay |44 operates in this circuit.

Ii the calling line had been line B which terminates on the corresponding level in the lower terminal bank tested by the lower brush set ISS-|35, terminal |58v of the line would have thereon the potential 'drop due to the resistance |0|" established as a result lof the closure of the calling line loop, and this drop would have been applied to the control element of tube |39A via brush |33 and resistance |49, causing the tube` |39 Ito break down and a circuit to be completed from negative' battery through the winding of relay |43, the cathode and anode of the tube |39, No. 2 Vcontacts of relay |4| through resistance |48 to positive battery. Relay |43 operates and completes a circuit for relay |4| via ground through the contacts of relay |43, No. 3 contacts of relay |42, winding of relay |4| to battery. Relay |4| operates and closes a locking path for itself through its No. 5 contacts to the sleeve conductor |02 to which ground will be connected by the iirst selector S` to hold said relay before the circuit of relay |43 is disrupted. Over its No. 3 and No. l front contacts, relay |4| transfers the line conductors |50 and |5| from the upper brushes |24 and |25, respectively, to the lower ybrushes |35 and |38, respectively, while the remaining functions of relay Ml, when operated, are the same as those of relay |42 which will be described shortly.

It' has been assumed that the calling line A appears in the fourth level oi the upper terminal bank. Therefore, rela-y |4| will not operate as above described under the assumption of a call being originated by line B but relay |42 operates in the manner described, causing the operation of relay |21 and the arresting of any further rotary operations as previously stated.

By the closure of its No. 2 contacts, relay I 42 applies negative battery through low resistance |52 to brush |22 and terminal H8 via the No. 1 back contacts of relay UH to establish a rbusy test to other hunting line finders, While, through its No. 8 contacts, it connects ground to brush |23 and terminal i9 via the No. 6 back contacts of relay |4|. rl'his ground is applied to the midpoint of high resistance |05 and short-circuits anode battery |01 connected to the line tube |05 causing 'both this tube and tube |02 to be reset. Relay 42 further connects line conductors |50 and |5I through its No. 4 and No. 5 contacts with conductors |53 and |54, respectively, which extend to the first selector S; at its No. 7 contacts it further removes positive battery from the anode of tube |40 causing this tube to be extinguished, in turn causing the release of relay |44 which, however, does not cause the release of relay |42 inasmuch as this relay ls now held to ground connected to conductor |62 by the rst selector S.

As already mentioned, the operationv of relay |42 extends` the line loop to the first selector S wherein a relay (not shown) bridged across conductors |53 and |54 and supplied with a source of current operates over the loop to cause the operation of another relay which connects ground to conductor |02 to hold relay |42 and also relay |4| if the calling line should happen to be located on' the lower terminal bank as already explained.

Under the control of dial impulses from the calling station A, the rst selector S- and, possibly, a second selector (not shown) as well as a connector C function to complete the connection to the wanted subscribers line D.V Since these operations are well known and fonn no part of the invention, they are not described.

When line tube |02 is extinguished, ionizing potential is `removed from the control element |56 of the group tube |08. When therefore, the low frequency source H5 causes the induction thereafter of a negative potential in the secondary winding of transformer i, the potential of anode H0 is, of course, negative and the tube |08 ceases conducting whereupon, after a time interval for discharging condenser |1, relay I2 releases to remove ground from segment |38 ofv commutator |26. The tube |08 is now normal and ready to reoperate when any one of its control electrodes is charged to a positive -potential by the closure of the line loop connected tol said control electrode. 'I'he tube lthen functions to operate relay H2 for the starting of another line finder as described.

When the call is completed and the subscriber on line A restores the receiver, this action causes the release of the selector train in the known manner and the consequent removal of ground from conductor` |'2 whereupon relay |42 releases. The release of relay M2 completes a circuit path for release magnet |55 extending from ground on the upper Contact set of Vertical off-normal springs VON which are closed as soon as the brush shaft moves off-normal, No. 3 contacts of relay |28, No. 6 contacts of relay |42, bottom contacts of relay |32, lower set of springs on the vertical onf-normal springs VON, winding of release magnet |55 t0 battery. Magnet |55 operates and restores the brush shaft to i-ts normal position.

While I have described my invention in connection with its specic application to an automatic line operating in conjunction with a known switch structure, it is to be understood that various other applications and embodiments thereof may be made by those skilled in the art Without departing from the spirit of the invention as denned Within the scope of the appended claims.

What is claimed is:

1. In a telephone system, a subscribers line, two space discharge devices for said line, means responsive to the initiation of a call from said line for discharging said devices, and means responsive to the discharge of said devices for producing a plurality of signaling potentials.

2. In a telephone system, a subscribers line terminating in a group of terminals, a pair of space discharge devices for said line, one of said devices being connected to one of said terminals and one side of said line, means responsive to the initiation of a call from said line for discharging said devices to produce a current flow through said line and through said devices whereby said means are effective to apply a test potential on a terminal of said group of tenninals, and other means connected to said terminal responsive to said current flow for producing a signaling potential.

3. In a telephone system, the combination with a switch and a telephone line connected to the terminals of said switch, said switch being adapted to operate in response to one potential for hunting for said terminals and to engage said terminals in response to another potential, of a pair oi space discharge devices for said line, means responsive to a call initiated over said line for discharging said pair of devices to produce both of said potentials, and means in said switch responsive to said iirst potential :for resetting said pair o devices upon the connection of said switch with said line terminals.

a. The combination in a telephone system of lines, a pair of space discharge devices for each of said lines adapted to discharge upon the initiation or a call from the associated line, another discharge device common to said lines and having as many control electrodes as there are lines, said electrodes being individual to said lines respectively and connected in operative relation to the pair of discharge devices of the associated lines, and switching means responsive to said other discharge device.

5. In a telephone system, the combination of subscribers lines, a pair of space discharge devices for each oi said lines adapted to discharge upon the initiation of a call thereover, another `altering the electrical condition of the electrode in said common device assigned to the calling line, a discharge circuit for said common device and current responsive means in said discharge circuit.

6. In a telephone system, the combination of subscribers lines, a pair of space discharge devices for each of said line-s, a space discharge device coinmon to said lines, a discharge circuit for said common discharge device, said common device having as many control electrodes as there are lines, an electrical network connecting each pair of line discharge devices with each control electrode respectively, means operative thro-ugh each of said networks for maintaining a potential on each of said electrodes to prevent a flow of current in said common discharge circuit, means responsive to a call initiated on one of said lines for operating the associated pair of discharge devices whereby the potential on the control electrode in the common device associated with said line is altered through the connecting network, and means inV said discharge circuit responsive to the flow of current therein.

7. In a telephone system, subscribers lines, a pair` of space discharge devices for each of said lines, a space discharge device common to said lines, said common device having a plurality of control elements individual respectively to said lines, a line-finder switch having a terminal bank in which said lines appear, means responsive to a call initiated from a line for operating the pair of discharge devices associated therewith, means responsive to said means for altering the potential of the corresponding control electrode in said common device to cause the discharge thereof, other means responsive to said rst-mentioned means for altering the potentialoi a corresponding terminal in said iinder switch, and means responsive to the discharge of said common device for setting said iinder switch in opieration to seize the terminals of said calling line.

8. The combination with a plurality of lines, of a gas-nlled tube having a plurality of control elements, a cathode and an anode, said elements relating respectively to said lines, means responsive to one of said lines when calling for altering the electrical condition of one of said elements, a transformer having one side of its secondary winding connected to the anode of said tube and the other side to a negative source of potential, a source of alternating voltage connected to the primary winding of said transformer whereby, in response to the altered condition of one of said electrodes, the voltage induced in the secondary winding of said transformer combined with a negative voltage from said negative source of potential connected thereto causes said tube to repeatedly and alternately discharge and quench, and means operative in response to the discharge of said tube.

9. In a telephone system, a group of lines, a space discharge tube having a plurality of control electrodes individual respectively to said lines, a space circuit for said tube, a line-finder switch having a terminal bank in which said lines appear, means for maintaining a given potential on each of said control electrodes and on each of the terminals of said lines in said nder switch, space discharge devices operative over the conductors of a calling one of said lines for altering the potential on the corresponding control electrode to cause the discharge of said tube and for altering the potential on the corresponding terminal of said line in the nder switch, a relay in said space circuit responsive to the discharge of said tube, means controlled by said relay for starting said finder switch in operation. test means rendered effective when the nder switch reaches the terminals of the calling line, means responsive to said test means for stopping the movement of said switch, and means effective upon the operation of said means for quenching said space discharge devices.

LUTHER G. SCHIMPF. 

